1. Field of the Invention
The present invention relates to imaging lenses which form an image of an object on a solid-state image sensor such as a CCD sensor or a C-MOS sensor used in a compact image pickup device, and more particularly to imaging lenses which are built in image pickup devices mounted in increasingly compact and low-profile mobile terminals such as smartphones, mobile phones and PDAs (Personal Digital Assistants), game consoles, information terminals such as PCs, and home appliances and wearable devices.
2. Description of the Related Art
In recent years, the market of compact image pickup devices has been ever-expanding with the spread of the use of image pickup devices in mobile terminals including smartphones, various home appliances and more recently so-called wearable information terminals. The cameras mounted in these products are expected to deliver high performance to cope with an increase in the number of pixels and at the same time be more compact and offer a wide field of view so as to be applicable to various types of products and provide high resolution and high brightness to cope with high-pixel image sensors.
As imaging lenses which meet the trend toward higher performance as mentioned above, many types of imaging lens composed of four constituent lenses (four-element imaging lenses) have been proposed, because an imaging lens comprised of four constituent lenses can be relatively compact and deliver high performance and can be supplied at low cost.
For example, JP-A-2008-033327 (Patent Document 1) discloses an imaging lens which includes a first lens with positive refractive power as a biconvex lens, a second lens with negative refractive power as a meniscus lens having a convex surface on an object side, a third lens with positive refractive power as a meniscus lens having a convex surface on an image side, and a fourth lens with negative refractive power, in which the length of the overall optical system in the optical axis direction and the ratio of the focal length of the first lens to the total focal length are set within appropriate ranges in order to achieve compactness and high performance.
JP-A-2009-265245 (Patent Document 2) discloses an imaging lens which includes, in order from an object side, a first lens with positive refractive power having a convex surface on the object side, an aperture stop for adjusting the quantity of light, a second lens with negative refractive power, a third lens with positive refractive power having a concave surface on the object side, and a fourth lens with negative refractive power, in which the ratio between the focal length of the overall optical system and the focal length of the second lens and the ratio between the focal length of the second lens and the curvature radius of the object-side surface of the second lens are set within appropriate ranges in order to achieve compactness and high performance.
In the imaging lens described in Patent Document 1, not only its configuration as mentioned above but also the use of glass material for the second lens contribute to its compactness and high performance. In addition, the ratio of the total track length to the diagonal length of the effective imaging plane of the image sensor (hereinafter referred to as the “total length to diagonal ratio”) is about 1.0, suggesting that the imaging lens is relatively compact. However, its F-value is about 3.0, so its brightness is not considered to be sufficient for an image sensor which deals with an increasing number of pixels. Also, its half field of view is in the range of 31 to 32 degrees, which is insufficient to meet the recent demand for a wide field of view.
The total length to diagonal ratio of the imaging lens described in Patent Document 2 is about 1.0, suggesting that the imaging lens is relatively compact. However, its F-value is about 2.8, so its brightness is not considered to be sufficient for an image sensor which deals with an increasing number of pixels. Also, its half field of view is in the range of 31 to 32 degrees, which is insufficient to meet the recent demand for a wide field of view.